Wavelength-specific incoupling gratings for display optimization

ABSTRACT

A head-mounted display (HMD) system includes a micro-display configured to project light beams, each of light beams encompassing a range of wavelengths different from each of the other light beams, and a waveguide having multiple incouplers configured to receive and direct light into the waveguide. Each of the incouplers is a diffraction grating with a fill factor different from each of the other incouplers. In some embodiments, each of the incouplers has a different period value from each of the other incouplers and the period value of each of the incouplers is based on the range of wavelengths of light each of the incouplers is configured to receive.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority to U.S. Provisional Patent Application Ser. No. 63/081,703, entitled “WAVEGUIDES WITH IMPROVED ANTIREFLECTIVE AND/OR COLOR RESPONSE PROPERTIES” and filed on Sep. 22, 2020, the entirety of which is incorporated by reference herein.

BACKGROUND

In a conventional wearable head-mounted display (HMD), light beams from an image source are coupled into a light guide substrate, generally referred to as a waveguide, by an input optical coupling such as an in-coupling diffraction grating (i.e., an “incoupler”), which can be formed on a surface, or multiple surfaces, of the substrate or disposed within the substrate. Once the light beams have been coupled into the waveguide, the light beams are “guided” through the substrate, typically by multiple instances of total internal reflection (TIR), to then be directed out of the waveguide by an output optical coupling (i.e., an “outcoupler”), which can also take the form of a diffractive optic. The light beams ejected from the waveguide overlap at an eye relief distance from the waveguide forming an exit pupil within which a virtual image generated by the image source can be viewed by the user of the HMD.

In some HMDs, the incoupler is a diffraction grating, which can be produced by physically forming grooves on a reflective surface. The overall efficiency of a ruled diffraction grating depends on various application-specific parameters such as wavelength, polarization, and angle of incidence of the incoming light. The efficiency of a ruled diffraction grating is also affected by the grating design parameters, such as the distance between adjacent grooves and the angle they form with the substrate. The quality of the virtual image output from a waveguide of an HMD is determined, at least in part, on the amount of light and the angle of the light that is directed into the waveguide by the incoupler. Thus, it is desirable to maximize the efficiency of a diffraction grating acting as an incoupler in an HMD to maximize the quality of the images projected by the HMD.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure may be better understood, and its numerous features and advantages made apparent to those skilled in the art by referencing the accompanying drawings. The use of the same reference symbols in different drawings indicates similar or identical items.

FIG. 1 illustrates an example display system in accordance with some embodiments.

FIG. 2 illustrates a block diagram of a laser projection system of the display system of FIG. 1, which projects images directly onto the eye of a user via laser light in accordance with some embodiments.

FIG. 3A shows an example of light propagation within a waveguide of the laser projection system of FIG. 2 in accordance with some embodiments.

FIG. 3B shows an example of a groove profile shape of an incoupler having a slant angle α, in accordance with some embodiments.

FIG. 4 shows an embodiment of a waveguide of a display system, such as the display system shown in FIG. 1, in which the incouplers are aligned horizontally in accordance with some embodiments.

FIG. 5 shows an embodiment of a waveguide of a display system, such as the display system shown in FIG. 1, in which the incouplers are aligned vertically in accordance with some embodiments.

FIG. 6 shows an embodiment of a waveguide of a display system, such as the display system shown in FIG. 1, in which the incouplers are staggered in accordance with some embodiments.

DETAILED DESCRIPTION

In some cases, the incoupler of a waveguide within an HMD is implemented as a diffraction grating (“grating”) disposed at a surface of the waveguide, wherein the diffraction grating diffracts different wavelengths of the light provided by a micro-display of the HMD into possibly different orders and at different angles into the waveguide. A diffraction grating is generally a surface having multiple features, such as groves or slits, formed thereon, and can be transmissive (e.g., multi-slit aperture) or reflective.

In general, a ruled diffraction grating consists of a number of parallel grooves with a groove spacing (p) (i.e., period), which is commonly reported as the groove density (G) (i.e., reciprocal of dG). Typical diffraction gratings have G values between 30 and 5000 grooves per millimeter. The groove spacing determines the angles at which a single wavelength of light will constructively interfere to form diffracted orders of light output from the diffraction grating. The efficiency of a diffraction grating is the amount of the incident light that is diffracted into a given diffraction order. The grating efficiency is a function of wavelength and polarization of the incident light, along with groove spacing, the fill factor, the shape of the grooves (i.e., groove profile), and the grating material. To optimize the efficiency of a grating for a particular wavelength, adjustments can be made to the groove profile, including facet angles, shapes, and/or depths to maximize the efficiency of the grating for the desired wavelength of light, which, consequently, reduces the efficiency of the grating for other wavelengths of light incident thereon.

Due to size constraints, some HMDs include only one diffraction grating incoupler to direct various wavelengths of light from the micro-display into the waveguide. Thus, to accommodate the wide range of wavelengths of light projected from the micro-display, the single incoupler is configured to provide moderate efficiency across the range of wavelengths, or the incoupler is configured to provide maximum efficiency for one specific wavelength at the expense of efficiency for the other wavelengths. In either case, there is a loss in the intensity and uniformity of images projected by the HMD. To avoid these losses and maximize efficiency, an HMD may include multiple waveguides, each having an incoupler optimized for a specific wavelength. While multiple waveguides each having a color-specific (i.e., wavelength-specific) incoupler allow for maximizing the efficiency and field of view of incoupling light into each waveguide, they also add bulk and weight to the device which detracts from the user experience.

Maximizing the efficiency of incoupling multiple wavelengths of light into a single waveguide can be achieved through the use of multiple spatially separated diffraction grating incouplers to allow for tuning of the grating characteristics to optimize the efficiency of each incoupler for a specific wavelength or range of wavelengths. That is, the groove spacing, facet angles, fill factors, and/or depths of the grooves of each incoupler can be optimized to maximize the efficiency of the grating for a specific color of light or range of colors. Also, depending on design considerations and constraints, multiple incouplers can be arranged in various configurations on or within the waveguide.

FIGS. 1-6 illustrate embodiments of systems and techniques to maximize the efficiency of incoupling light into a waveguide of a display system utilizing multiple incouplers, as described in greater detail below. However, it will be appreciated that the apparatuses and techniques of the present disclosure are not limited to implementation in this particular display system, but instead may be implemented in any of a variety of display systems using the guidelines provided herein.

FIG. 1 illustrates an example display system 100 having a support structure 102 that includes an arm 104, which houses a laser projection system configured to project images toward the eye of a user, such that the user perceives the projected images as being displayed in a field of view (FOV) area 106 of a display at one or both of lens elements 108, 110. In the depicted embodiment, the display system 100 is a wearable head-mounted display (HMD) that includes a support structure 102 configured to be worn on the head of a user and has a general shape and appearance of an eyeglasses frame. The support structure 102 contains or otherwise includes various components to facilitate the projection of such images toward the eye of the user, such as a laser projector, an optical scanner, and a waveguide. In some embodiments, the support structure 102 further includes various sensors, such as one or more front-facing cameras, rear-facing cameras, other light sensors, motion sensors, accelerometers, and the like. The support structure 102 further can include one or more radio frequency (RF) interfaces or other wireless interfaces, such as a Bluetooth™ interface, a WiFi interface, and the like. Further, in some embodiments, the support structure 102 includes one or more batteries or other portable power sources for supplying power to the electrical components of the display system 100. In some embodiments, some or all of these components of the display system 100 are fully or partially contained within an inner volume of support structure 102, such as within the arm 104 in region 112 of the support structure 102. It should be noted that while an example form factor is depicted, it will be appreciated that in other embodiments the display system 100 may have a different shape and appearance from the eyeglasses frame depicted in FIG. 1.

One or both of the lens elements 108, 110 are used by the display system 100 to provide an augmented reality (AR) or mixed reality (MR) display in which rendered graphical content can be superimposed over or otherwise provided in conjunction with a real-world view as perceived by the user through the lens elements 108, 110. For example, laser light used to form a perceptible image or series of images may be projected by a laser projector of the display system 100 onto the eye of the user via a series of optical elements, such as a waveguide formed at least partially in the corresponding lens element, one or more scan mirrors, and one or more optical relays. One or both of the lens elements 108, 110 thus include at least a portion of a waveguide that routes display light received by an incoupler, or multiple incouplers, of the waveguide to an outcoupler of the waveguide, which outputs the display light toward an eye of a user of the display system 100. The display light is modulated and projected onto the eye of the user such that the user perceives the display light as an image. In addition, each of the lens elements 108, 110 is sufficiently transparent to allow a user to see through the lens elements to provide a field of view of the user's real-world environment such that the image appears superimposed over at least a portion of the real-world environment.

In some embodiments, the projector is a matrix-based projector, a scanning laser projector, or any combination of a modulative light source such as a laser or one or more LEDs and a dynamic reflector mechanism such as one or more dynamic scanners or digital light processors. In some embodiments, the projector includes multiple laser diodes (e.g., a red laser diode, a green laser diode, and/or a blue laser diode) and at least one scan mirror (e.g., two one-dimensional scan mirrors, which may be micro-electromechanical system (MEMS)-based or piezo-based). The projector is communicatively coupled to the controller and a non-transitory processor-readable storage medium or memory storing processor-executable instructions and other data that, when executed by the controller, cause the controller to control the operation of the projector. In some embodiments, the controller controls a scan area size and scan area location for the projector and is communicatively coupled to a processor (not shown) that generates content to be displayed at the display system 100. The projector scans light over a variable area, designated the FOV area 106, of the display system 100. The scan area size corresponds to the size of the FOV area 106 and the scan area location corresponds to a region of one of the lens elements 108, 110 at which the FOV area 106 is visible to the user. Generally, it is desirable for a display to have a wide FOV to accommodate the outcoupling of light across a wide range of angles. Herein, the range of different user eye positions that will be able to see the display is referred to as the eyebox of the display.

In some embodiments, the projector routes light via first and second scan mirrors, an optical relay disposed between the first and second scan mirrors, and a waveguide disposed at the output of the second scan mirror. In some embodiments, at least a portion of an outcoupler of the waveguide may overlap the FOV area 106. These aspects are described in greater detail below.

FIG. 2 illustrates a block diagram of a laser projection system 200 that projects images directly onto the eye of a user via laser light. The laser projection system 200 includes an optical engine 202, an optical scanner 204, and a waveguide 205. The optical scanner 204 includes a first scan mirror 206, a second scan mirror 208, and an optical relay 210. The waveguide 205 includes three incouplers 212-1, 212-2, and 212-3 (collectively and generically 212) positioned to receive light from the second scan mirror 208. Each of incouplers 212-1, 212-2, and 212-3 is optimized to receive and diffract light of a specific wavelength, or range of wavelengths, into the waveguide 205. While FIG. 2 shows three incouplers, it should be understood that any number of incouplers can be provided on the waveguide 205.

The waveguide also includes an outcoupler 214, with the outcoupler 214 being optically aligned with an eye 216 of a user in the present example. In some embodiments, the laser projection system 200 is implemented in a wearable heads-up display or other display system, such as the display system 100 of FIG. 1. While outcoupler 214 is shown and described as a single feature in FIG. 2, it should be understood that outcoupler 214 may be a collection of individual features disposed within a region of the waveguide 205 and configured to direct light outward from the waveguide 205, generally towards a user's eye.

The optical engine 202 includes one or more laser light sources configured to generate and output laser light 218 of a specific wavelength or range of wavelengths (e.g., visible laser light such as red, blue, and green laser light and/or non-visible laser light such as infrared laser light). In some embodiments, the optical engine 202 is coupled to a driver or other controller (not shown), which controls the timing of emission of laser light from the laser light sources of the optical engine 202 in accordance with instructions received by the controller or driver from a computer processor coupled thereto to modulate the laser light 218 to be perceived as images when output to the retina of an eye 216 of a user.

For example, during the operation of the laser projection system 200, multiple laser light beams, each having a different wavelength or range of wavelengths, are output by the laser light sources of the optical engine 202, and each laser light beam is directed by the optical scanner 204 to one of incouplers 212-1, 212-2, or 212-3 based on the properties of the respective incoupler. That is, in some embodiments, the characteristics of incoupler 212-1 are configured to diffract light within a range of wavelengths between 750 nm and 590 nm, which generally corresponds to red light, with maximum efficiency. Further, the characteristics of incoupler 212-2 are optimally configured to diffract laser light within a range of wavelengths between 591 nm and 495 nm, which generally corresponds to green light, and the characteristics of incoupler 212-3 are optimally configured to diffract laser light within a range of wavelengths between 496 nm and 380 nm, which generally corresponds to blue light.

The optical engine 202 modulates the respective intensities of the laser light beams so that the combined laser light reflected at the outcoupler 214 projects a series of pixels of an image, with the particular intensity of each laser light beam at any given point in time contributing to the amount of corresponding color content and brightness in the pixel being represented by the combined laser light at that time.

One or both of the scan mirrors 206 and 208 of the optical scanner 204 are MEMS mirrors in some embodiments. For example, the scan mirror 206 and the scan mirror 208 are MEMS mirrors that are driven by respective actuation voltages to oscillate during active operation of the laser projection system 200, causing the scan mirrors 206 and 208 to scan the laser light 218. Oscillation of the scan mirror 206 causes laser light 218 output by the optical engine 202 to be scanned through the optical relay 210 and across a surface of the second scan mirror 208. The second scan mirror 208 scans the laser light 218 received from the scan mirror 206 towards one of the incouplers 212 of the waveguide 205. In some embodiments, the scan mirror 206 oscillates along a first scanning axis 219, such that the laser light 218 is scanned in only one dimension (i.e., in a line) across the surface of the second scan mirror 208. In some embodiments, the scan mirror 208 oscillates or otherwise rotates along a second scanning axis 221. In some embodiments, the first scanning axis 219 is perpendicular to the second scanning axis 221.

In some embodiments, the incouplers 212 have a substantially rectangular profile and are configured to receive the laser light 218 and direct the laser light 218 into the waveguide 205. The incouplers 212 are defined by a smaller dimension (i.e., width) and a larger orthogonal dimension (i.e., length). In an embodiment, the optical relay 210 is a line-scan optical relay that receives the laser light 218 scanned in a first dimension by the first scan mirror 206 (e.g., the first dimension corresponding to the small dimension of the incoupler 212), routes the laser light 218 to the second scan mirror 208, and introduces a convergence to the laser light 218 in the first dimension to an exit pupil beyond the second scan mirror 208. Herein, an “exit pupil” in an optical system refers to the location along the optical path where beams of light intersect. For example, the possible optical paths of the laser light 218, following reflection by the first scan mirror 206, are initially spread along the first scanning axis, but later these paths intersect at an exit pupil beyond the second scan mirror 208 due to convergence introduced by the optical relay 210. For example, the width (i.e., smallest dimension) of a given exit pupil approximately corresponds to the diameter of the laser light corresponding to that exit pupil. Accordingly, the exit pupil can be considered a “virtual aperture”. According to various embodiments, the optical relay 210 includes one or more collimation lenses that shape and focus the laser light 218 on the second scan mirror 208 or includes a molded reflective relay that includes two or more spherical, aspheric, parabolic, and/or freeform lenses that shape and direct the laser light 218 onto the second scan mirror 208. The second scan mirror 208 receives the laser light 218 and scans the laser light 218 in a second dimension, the second dimension corresponding to the long dimension of one of the incouplers 212 of the waveguide 205. In some embodiments, the second scan mirror 208 causes the exit pupil of the laser light 218 to be swept along a line along the second dimension. In some embodiments, the incoupler 212 is positioned at or near the swept line downstream from the second scan mirror 208 such that the second scan mirror 208 scans the laser light 218 as a line or row over one of the incouplers 212.

The waveguide 205 of the laser projection system 200 includes the incouplers 212 and the outcoupler 214. The term “waveguide,” as used herein, will be understood to mean a combiner using one or more of total internal reflection (TIR), specialized filters, and/or reflective surfaces, to transfer light from an incoupler (such as one of incouplers 212-1, 212-2, 212-3) to an outcoupler (such as the outcoupler 214). In general, the terms “incoupler” and “outcoupler” will be understood to refer to any type of optical grating structure, including, but not limited to, diffraction gratings, holograms, holographic optical elements (e.g., optical elements using one or more holograms), volume diffraction gratings, volume holograms, surface relief diffraction gratings, and/or surface relief holograms. In some embodiments, a given incoupler or outcoupler is configured as a transmissive grating (e.g., a transmissive diffraction grating or a transmissive holographic grating) that causes the incoupler or outcoupler to transmit light and to apply designed optical function(s) to the light during the transmission. In some embodiments, a given incoupler or outcoupler is a reflective grating (e.g., a reflective diffraction grating or a reflective holographic grating) that causes the incoupler or outcoupler to reflect light and to apply designed optical function(s) to the light during the reflection. In the present example, the laser light 218 received at incouplers 212-1, 212-2, 212-3 is propagated to the outcoupler 214 via the waveguide 205 using TIR. The laser light 218 is then output to the eye 216 of a user via the outcoupler 214. As described above, in some embodiments the waveguide 205 is implemented as part of an eyeglass lens, such as the lens 108 or lens 110 (FIG. 1) of the display system having an eyeglass form factor and employing the laser projection system 200.

Although not shown in the example of FIG. 2, in some embodiments additional optical components are included in any of the optical paths between the optical engine 202 and the scan mirror 206, between the scan mirror 206 and the optical relay 210, between the optical relay 210 and the scan mirror 208, between the scan mirror 208 and the incoupler 212, between the incouplers 212 and the outcoupler 214, and/or between the outcoupler 214 and the eye 216 (e.g., in order to shape the laser light for viewing by the eye 216 of the user). In some embodiments, a prism is used to steer light from the scan mirror 208 into the incouplers 212 so that light is coupled into each of the incouplers 212 at the appropriate angle to encourage propagation of the light in waveguide 205 by TIR. Also, in some embodiments, an exit pupil expander (e.g., an exit pupil expander 304 of FIG. 3A, described below), such as a fold grating, is arranged in an intermediate stage between incoupler 212 and outcoupler 214 to receive light that is coupled into waveguide 205 by the incouplers 212, expand the light, and redirect the light towards the outcoupler 214, where the outcoupler 214 then couples the laser light out of waveguide 205 (e.g., toward the eye 216 of the user).

FIG. 3A shows an example of light propagation within the waveguide 205 of the laser projection system 200 of FIG. 2 in accordance with some embodiments. As shown, light received via incouplers 212-1, 212-2, 212-3, which is scanned along the axis 302, is directed into an exit pupil expander 304 and is then routed to the outcoupler 214 to be output from the waveguide 205 (e.g., toward the eye of the user). In some embodiments, the exit pupil expander 304 expands one or more dimensions of the eyebox of an HMD that includes the laser projection system 200 (e.g., with respect to what the dimensions of the eyebox of the HMD would be without the exit pupil expander 304). In some embodiments, the incouplers 212 and the exit pupil expander 304 each include respective one-dimensional diffraction gratings (i.e., diffraction gratings that extend along one dimension). It should be understood that FIG. 3A shows a substantially ideal case in which the incouplers 212 direct light straight down (with respect to the presently illustrated view) in a first direction that is perpendicular to the scanning axis 302, and the exit pupil expander 304 directs light to the right (with respect to the presently illustrated view) in a second direction that is perpendicular to the first direction. While not shown in the present example, it should be understood that, in some embodiments, the first direction in which the incouplers 212 direct light is slightly or substantially diagonal, rather than exactly perpendicular, with respect to the scanning axis 302.

Also shown in FIG. 3A is a cross-section 306 of incoupler 212-1 illustrating characteristics of the diffraction grating that can be configured to optimize the efficiency of incoupler 212-1 based on the wavelength of light that incoupler 212-1 is intended to receive and incouple to the waveguide 205. The period p of the grating is shown having two regions, with transmittances t₁=1 and t₂=0 and widths d₁ and d₂, respectively. The grating period is constant p=d₁+d₂, but the relative widths d₁, d₂ of the two regions may vary. A fill factor parameter x can be defined such that d₁=xp and d₂=(1−x)p. In addition, while the profile shape of the groove profile in cross-section 306 is generally shown as being square or rectangular with a height h, the shape can be modified based on the wavelength of light that incoupler 212-1 is intended to receive. For example, in some embodiments, the shape of the groove profile is triangular, rather than square, to create a more “saw-toothed” profile. As another example, the grooves can have a slant angle α, as measured from the normal line 308 to the surface in which the grooves are formed, as shown in FIG. 3B. The slant angle α of the grooves is tuned based on the wavelength of light the incoupler 212 is intended to receive. In some embodiments, incouplers 212-1, 212-2, 212-3 are configured as gratings with the same period but different fill factors, heights, and slant angles based on the wavelength of light the respective incoupler 212 is intended to receive. In some embodiments, each of incouplers 212-1, 212-2, 212-3 are configured as gratings with differing periods, fill factors, heights, and slant angles based on the wavelength of light the incoupler 212 is intended to receive. In some embodiments, each of incouplers 212-1, 212-2, 212-3 are selectively coated with a dielectric or metal coating to modify the diffraction efficiency of the grating based on the wavelength of light the incoupler 212 is intended to receive.

FIGS. 4-6 illustrate example placement locations for incouplers 212 within a waveguide, such as waveguide 205 of FIG. 2. FIG. 4 shows a waveguide 405 in which incouplers 212-1, 212-2, and 212-3 are aligned horizontally (i.e., side-by-side) along axis 302. Accordingly, the second scanning mirror 208 of the optical scanner 204 is configured to scan light of a first wavelength, or first range of wavelengths, across the width of incoupler 212-1 along axis 302. The second scanning mirror is also configured to scan light of a second wavelength, or second range of wavelengths, across the width of incoupler 212-2 along axis 302, and to scan light of a third wavelength, or third range of wavelengths, across the width of incoupler 212-3 along axis 302.

FIG. 5 shows a waveguide 505 in which incouplers 212-1, 212-2, and 212-3 are aligned vertically (i.e., stacked) such that each incoupler 212 has an individual axis 502, 504, 506 along which light is scanned, the individual axes 502, 504, 506 being parallel to one another. Accordingly, the second scanning mirror 208 of the optical scanner 204 is configured to scan light of a first wavelength, or first range of wavelengths, across the width of incoupler 212-1 along axis 502. The second scanning mirror is also configured to scan light of a second wavelength, or second range of wavelengths, across the width of incoupler 212-2 along axis 504, and to scan light of a third wavelength, or third range of wavelengths, across the width of incoupler 212-3 along axis 506.

FIG. 6 shows a waveguide 605 in which incouplers 212-1, 212-2, and 212-3 are disposed in a staggered configuration such that incoupler 212-2 is aligned on axis 602 and incouplers 212-1 and 212-3 are aligned horizontally along axis 604. Accordingly, the second scanning mirror 208 of the optical scanner 204 is configured to scan light of a first wavelength, or first range of wavelengths, across the width of incoupler 212-2 along axis 604. The second scanning mirror is also configured to scan light of a second wavelength, or second range of wavelengths, across the width of incoupler 212-1 along axis 602, and to scan light of a third wavelength, or third range of wavelengths, across the width of incoupler 212-3 along axis 602. While the incouplers 212 shown in FIGS. 4-6 are generally depicted as having similar dimensions and shapes, it should be understood that each of incouplers 212 may have different dimensions and/or shapes from the other incouplers.

In some embodiments, certain aspects of the techniques described above may be implemented by one or more processors of a processing system executing software. The software comprises one or more sets of executable instructions stored or otherwise tangibly embodied on a non-transitory computer-readable storage medium. The software can include the instructions and certain data that, when executed by the one or more processors, manipulate the one or more processors to perform one or more aspects of the techniques described above. The non-transitory computer-readable storage medium can include, for example, a magnetic or optical disk storage device, solid state storage devices such as Flash memory, a cache, random access memory (RAM), or other non-volatile memory device or devices, and the like. The executable instructions stored on the non-transitory computer-readable storage medium may be in source code, assembly language code, object code, or other instruction format that is interpreted or otherwise executable by one or more processors.

A computer-readable storage medium may include any storage medium, or combination of storage media, accessible by a computer system during use to provide instructions and/or data to the computer system. Such storage media can include, but is not limited to, optical media (e.g., compact disc (CD), digital versatile disc (DVD), Blu-ray disc), magnetic media (e.g., floppy disc, magnetic tape, or magnetic hard drive), volatile memory (e.g., random access memory (RAM) or cache), non-volatile memory (e.g., read-only memory (ROM) or Flash memory), or microelectromechanical systems (MEMS)-based storage media. The computer-readable storage medium may be embedded in the computing system (e.g., system RAM or ROM), fixedly attached to the computing system (e.g., a magnetic hard drive), removably attached to the computing system (e.g., an optical disc or Universal Serial Bus (USB)-based Flash memory), or coupled to the computer system via a wired or wireless network (e.g., network accessible storage (NAS)).

Note that not all of the activities or elements described above in the general description are required, that a portion of a specific activity or device may not be required, and that one or more further activities may be performed, or elements included, in addition to those described. Still further, the order in which activities are listed is not necessarily the order in which they are performed. Also, the concepts have been described with reference to specific embodiments. However, one of ordinary skill in the art appreciates that various modifications and changes can be made without departing from the scope of the present disclosure as set forth in the claims below. Accordingly, the specification and figures are to be regarded in an illustrative rather than a restrictive sense, and all such modifications are intended to be included within the scope of the present disclosure.

Benefits, other advantages, and solutions to problems have been described above with regard to specific embodiments. However, the benefits, advantages, solutions to problems, and any feature(s) that may cause any benefit, advantage, or solution to occur or become more pronounced are not to be construed as a critical, required, or essential feature of any or all the claims. Moreover, the particular embodiments disclosed above are illustrative only, as the disclosed subject matter may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. No limitations are intended to the details of construction or design herein shown, other than as described in the claims below. It is therefore evident that the particular embodiments disclosed above may be altered or modified and all such variations are considered within the scope of the disclosed subject matter. Accordingly, the protection sought herein is as set forth in the claims below. 

What is claimed is:
 1. A waveguide comprising: a plurality of incouplers configured to receive and direct light into the waveguide, wherein each of the plurality of incouplers is a diffraction grating with a fill factor different from each of the other plurality of incouplers.
 2. The waveguide of claim 1, wherein the plurality of incouplers have identical period values.
 3. The waveguide of claim 1, wherein each of the plurality of incouplers has a different period value from each of the other plurality of incouplers, the period value of each of the plurality of incouplers being based on a range of wavelengths of light each of the plurality of incouplers is configured to receive.
 4. The waveguide of claim 1, wherein each of the plurality of incouplers has a different grating height from each of the other plurality of incouplers, the grating height of each of the plurality of incouplers being based on the range of wavelengths of light each of the plurality of incouplers is configured to receive.
 5. The waveguide of claim 1, wherein the incouplers of the plurality of incouplers are aligned horizontally within the waveguide.
 6. The waveguide of claim 1, wherein the incouplers of the plurality of incouplers are aligned vertically within the waveguide.
 7. The waveguide of claim 1, wherein the incouplers of the plurality of incouplers are disposed in a staggered configuration within the waveguide.
 8. A head-mounted display (HMD) system comprising: a display configured to project a plurality of light beams, each of the plurality of light beams encompassing a range of wavelengths different from each of the other plurality of light beams; and a waveguide comprising a plurality of incouplers configured to receive and direct light into the waveguide, wherein each of the plurality of incouplers is a diffraction grating with a fill factor different from each of the other plurality of incouplers.
 9. The HMD system of claim 8, wherein the plurality of incouplers have identical period values.
 10. The HMD system of claim 8, wherein each of the plurality of incouplers has a different period value from each of the other plurality of incouplers, the period value of each of the plurality of incouplers being based on the range of wavelengths of light each of the plurality of incouplers is configured to receive.
 11. The HMD system of claim 8, wherein each of the plurality of incouplers has a different grating height from each of the other plurality of incouplers, the grating height of each of the plurality of incouplers being based on the range of wavelengths of light each of the plurality of incouplers is configured to receive.
 12. The HMD system of claim 8, wherein the incouplers of the plurality of incouplers are aligned horizontally within the waveguide.
 13. The HMD system of claim 8, wherein the incouplers of the plurality of incouplers are aligned vertically within the waveguide.
 14. The HMD system of claim 8, wherein the incouplers of the plurality of incouplers are disposed in a staggered configuration within the waveguide.
 15. A method comprising: receiving light within a first range of wavelengths at a first incoupler of a waveguide of a head-mounted display (HMD); and receiving light with a second range of wavelengths, different from the first range of wavelengths, at a second incoupler of the HMD, the second incoupler having a fill factor different than a fill factor of the first incoupler.
 16. The method of claim 15, wherein the first incoupler and the second incoupler have identical period values; and wherein the first incoupler has a different grating height from the second incoupler, the grating height of the first and second incouplers being based on a range of wavelengths of light each of the first and second incouplers receives.
 17. The method of claim 15, wherein the first incoupler has a different period value from the second incoupler, the period value of the first and second incouplers being based on the range of wavelengths of light each of the first and second incouplers receives.
 18. The method of claim 15, wherein the first incoupler and second incoupler are aligned horizontally within the waveguide.
 19. The method of claim 15, wherein the first incoupler and second incoupler are aligned vertically within the waveguide.
 20. The method of claim 15, wherein the first incoupler and second incoupler are disposed in a staggered configuration within the waveguide. 